Methods for characterizing carbon overcoat

ABSTRACT

A method for characterizing a carbon overcoat is provided. The method includes performing electron energy loss spectroscopy (EELS) spectrum imaging for an area of a preselected carbon-based material and an area of the carbon overcoat to generate a reference EELS dataset and a carbon overcoat EELS dataset, respectively, and determining a carbon bonding content of the carbon overcoat based on the reference EELS dataset and the carbon overcoat EELS dataset.

FIELD

Aspects of the present invention relate to characterization of carbonovercoat, and, in particular, to processes and methods forcharacterizing ultra thin carbon overcoat.

BACKGROUND

With the continued scaling down of the magnetic head used in harddrives, improved magnetic signal-to-noise ratio in the magnetic head andmedia is needed for a new generation of hard drives. To improveperformance, ultra thin (e.g., less than about 3 nm) carbon overcoat(COC) has been used in the fabrication of new generations of head andmedia. The carbon overcoat is fabricated to achieve the desired chemicalstate in order to ensure the mechanical and/or thermal properties neededfor the prescribed performance specifications of the head and media.This can be partially achieved by using unpatterned full (or thick) filmmonitor coupons, that are used to monitor a workpiece wafer. The couponis a separate wafer such as a silicon wafer that is processed insubstantially the same way as the workpiece wafer.

However, the generally known characterization methods of thick filmsbecomes unreliable when the thickness of the carbon overcoat is lessthan about 3 nm. It is because thin carbon overcoat is intrinsically andchemically different from thick films. Therefore, conventionalcharacterization methods applied to thick films become unreliable andless sensitive in measuring thickness, composition, and chemical bondingwhen the film thickness drops below about 3 nm. Additionally, the carbonovercoat at different locations of a head has different propertiesdepending on the areas, such as substrate, shield, where the carbonovercoat is grown on. Therefore, a reliable technique is needed to fullycharacterize the carbon overcoat used in head and media development andmanufacturing processes.

Various methods have been used to characterize the chemical bondinginformation (sp³/sp² ratio) of carbon overcoat films, such as Ramanspectroscopy, solid-state nuclear magnetic resonance (NMR), X-rayphotoelectron spectroscopy (XPS), and electron energy loss spectroscopy(EELS) in transmission electron microscopy (TEM). While Raman, NMR, andXPS are useful techniques, the carbon overcoat generally needs to have athickness more than about 3 nm to carry out the measurement with areasonable signal-to-noise ratio. While EELS can work on thinner filmsdown to sub-nanometer in thickness, there is no known direct way todetect a sp³/sp² ratio of a carbon overcoat (COC) using EELS.

SUMMARY

Embodiments of the present invention are directed to methods for fullycharacterizing an ultra thin (e.g., less than about 3 nm) carbonovercoat. Full characterization of the carbon overcoat includesdetermining the thickness of the carbon overcoat, which generallyincludes a carbon layer and a seed layer, the composition profile andtwo-dimensional map of the carbon overcoat, and the carbon chemicalbonding sp³/sp² ratio of the carbon overcoat.

A method for characterizing a carbon overcoat is provided according toone embodiment of the present invention. The method include: performingelectron energy loss spectroscopy (EELS) spectrum imaging for an area ofa preselected carbon-based reference material and an area of the carbonovercoat to generate a reference EELS dataset and a carbon overcoat EELSdataset, respectively; and determining a carbon bonding content of thecarbon overcoat based on the reference EELS dataset and the carbonovercoat EELS dataset.

A method for characterizing a carbon overcoat is provided according toanother embodiment of the present invention. The method includes:measuring a thickness of the carbon overcoat using transmission electronmicroscopy (TEM); measuring a thickness of a sub-layer of the carbonovercoat using energy filtered transmission electron microscopy (EFTEM)or scanning transmission electron microscopy (STEM), the sub-layercomprising carbon and a seed material; performing electron energy lossspectroscopy (EELS) spectrum imaging for an area of a preselectedcarbon-based material and an area of the carbon overcoat to generate areference EELS dataset and a carbon overcoat EELS dataset; anddetermining a carbon bonding of the carbon overcoat based on thereference EELS dataset and the carbon overcoat EELS dataset, the carbonbonding selected from the group consisting of a sp³ bonding and a sp²bonding.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and aspects of the present invention willbecome more apparent by describing in detail embodiments thereof withreference to the attached drawings in which:

FIG. 1 is a graph illustrating electron energy loss spectroscopyspectrum for a carbon overcoat, the graph being fitted with curves fordetermining the pi orbital and sigma orbital peak intensities accordingto an embodiment of the present invention;

FIG. 2 is a flowchart illustrating a number of processes for fullycharacterizing a carbon overcoat according to an embodiment of thepresent invention; and

FIG. 3 is a flowchart illustrating a method for characterizing a carbonovercoat according to an embodiment of the present invention.

DETAILED DESCRIPTION

Carbon can form a great variety of crystalline and disordered structuresbecause carbon can exist in three hybridization states such as sp³, sp²,and sp¹. There is no known direct way to detect a sp³/sp² ratio of acarbon overcoat (COC). However, a theoretical model has been known thatcan be used to calculate the sp³/sp² ratio of a carbon overcoat fromelectron energy loss spectroscopy (EELS) of the carbon overcoat.However, the theoretical model has the following limitations: 1) it canproduce satisfactory results for unpatterned full film having a filmthickness of more than about 3 nm in order to carry out the EELSmeasurement with a good signal-to-noise ratio; 2) there are no knownprocedures to do the EELS measurement with good repeatability forpatterned devices having a film thickness of less than about 3 nm; and3) there is no known way to process the EELS data with good reliabilityand consistency. Embodiments of the present invention solve theabove-described problems by utilizing EELS in an innovative way toprovide a practical way of utilizing the theoretical model in real lifeapplications.

Currently there is no reliable method to characterize the chemical stateof an ultra thin (e.g., less than about 3 nm) carbon overcoat (e.g.,diamond-like carbon) of nanometers localized at patterned device level.Embodiments of the present invention provide a characterization methodfor quantitatively characterizing and comparing relative sp³/sp² ratioof a carbon overcoat directly on the nanometer sized device withsub-nanometer spatial resolution together with other information such asthickness and composition. This characterization method employs acombination of transmission electron microscopy (TEM), scanningtransmission electron microscopy (STEM), energy filtered transmissionelectron microscopy (EFTEM), and/or electron energy loss spectroscopy(EELS).

High energy resolution (e.g., about 1 eV) EELS can be used to evaluatethe sp³/sp² ratio of carbon overcoats for head and media. The evaluationof the sp³/sp² ratio can be accomplished by determining the relativeintensity of pi bond (π*) and sigma bond (σ*) at the carbon K-edge ofthe EELS spectra of the carbon overcoat, and comparing the relativeintensity to that of a preselected carbon-based reference material(hereafter “reference material”) such as a material constituted of 100percent sp² microcrystalline graphite reference or C₆₀. In variousembodiments of the present invention, a number of procedures/processesare performed to characterize a carbon overcoat such as a patternedultra thin (e.g., between about 1 nm to about 3 nm, inclusive). Thecharacterization includes determining the thickness, composition,chemical bonds, and sp³/sp² ratio of the carbon overcoat based on thetheoretical model to be described below in more detail.

Application of Theoretical Model

FIG. 1 is a graph illustrating electron energy loss spectroscopyspectrum for a carbon overcoat, the graph being fitted with curves(e.g., Gaussian curves) for determining the pi orbital and sigma orbitalpeak intensities according to an embodiment of the present invention. InFIG. 1, preselected energy windows are defined to fit a pi carbonbonding (π*) curve 10 and a sigma carbon bonding (σ*) curve 20,respectively, in order to determine the pi orbital intensity I_(π) andsigma orbital intensity I_(σ) through integration under the respectivecurves. The same processes are also performed on a reference material(e.g., 100 percent sp² carbon-based material). To a good approximation,the ratio of the integrated areas under the energy windows (e.g., 284 eVto 289 eV for I_(π) and 290 eV to 305 eV for I_(σ)) is proportional toN_(π)/N_(σ), which is a ratio of the number of π and σ orbitals. Theratio N_(π)/N_(σ) is 1/3 for 100 percent sp² bonded carbon and 0/4 for100 percent sp³ bonded carbon. Therefore, the number of sp³ bondedcarbon atoms can be expressed as N(sp³)=(N_(σ)−3N_(π))/4, and the numberof sp² bonded carbon atoms can be expressed as N(sp²)=N_(π).Accordingly, using a reference that contains 100 percent sp² bondedcarbon atoms, the number fraction F(sp³) of sp³ bonded atoms and thenumber fraction F(sp²) of sp² bonded atoms of the carbon overcoat (COC)can be determined by Equations (1) and (2).

$\begin{matrix}{{{{F\left( {sp}^{3} \right)} = \frac{1 - {3 \cdot \frac{N_{\pi}}{N_{\sigma}}}}{1 + \frac{N_{\pi}}{N_{\sigma}}}},{where}}{\frac{N_{\pi}}{N_{\sigma}} = \frac{\left\lbrack {I_{\pi}/I_{\sigma}} \right\rbrack_{coc}}{3 \cdot \left\lbrack {I_{\pi}/I_{\sigma}} \right\rbrack_{reference}}}} & {{Equation}\mspace{14mu}(1)}\end{matrix}$

$\begin{matrix}{{F\left( {sp}^{2} \right)} = {1 - {F\left( {sp}^{3} \right)}}} & {{Equation}\mspace{14mu}(2)}\end{matrix}$

Here, N_(π) is a number of carbon atoms having the sp² bonding, andN_(σ) is a number of carbon atoms having the sp³ bonding. I_(π) is a piorbital intensity, and I_(σ) is a sigma orbital intensity, at a carbonK-edge. Therefore, using a reference material of 100 percent sp² bondedcarbon atoms, the sp³/sp² ratio of the carbon overcoat can be determinedbased on Equations (1) and (2).

In several embodiments, the carbon overcoat can include other elements(e.g., N, H, or O) in addition to carbon, and the carbon overcoat willhave bondings between carbon and the other elements. Therefore,additional Gaussian curves can be fitted to account for thenon-carbon-to-carbon bondings, and the corresponding I_(π) and I_(σ) ofthe non-carbon-carbon bondings can be subtracted. In such case,Equations (1) and (2) can still be used to determine the desired sp³/sp²ratio of carbon-to-carbon bonding of the carbon overcoat.

Characterization Processes

FIG. 2 is a flowchart illustrating a number of processes for fullycharacterizing a carbon overcoat (e.g., a diamond-like carbon coating)according to an embodiment of the present invention. However, thepresent invention is not limited thereto. In various embodiments, theprocesses can be performed in different orders. In several embodiments,some of the processes can be omitted. In other embodiments, additionalprocesses can be performed. In several embodiments, the processes can beapplied to characterize a carbon overcoat of a magnetic head at wafer,slider, and head gimbal assembly (HGA) level. In one embodiment, theprocesses can be used to characterize the carbon overcoat of magneticmedia.

To fully characterize a carbon overcoat, the processes determine thethickness of the carbon overcoat including the thickness of a carbonlayer and a seed layer of the carbon overcoat, the composition profileand two-dimensional map of the carbon overcoat, and the carbon chemicalbonding sp³/sp² ratio of the carbon overcoat. The processes will bedescribed in more detail in reference to FIG. 2 as a non-limitingexample. Referring to block S1 of FIG. 2, a thickness of a carbonovercoat is measured using a microscopy technique such as transmissionelectron microscopy (TEM) and scanning transmission electron microscopy(STEM). Further, a thickness of carbon and a seed material (a sub-layer)of the carbon overcoat is measured using a microscopy technique such asenergy filtered transmission electron microscopy (EFTEM) and STEM.

Referring to block S2 of FIG. 2, a preselected carbon-based material(hereafter “reference material”) is used as a reference forcharacterizing the carbon overcoat using electron energy lossspectroscopy (EELS). In several embodiments, the reference materialincludes substantially 100 percent sp² bonded carbon (e.g.,microcrystalline graphite) such that EELS orientation dependence effectcan be eliminated. For example, when the reference material is amicrocrystalline graphite, the electron density of such material issubstantially the same in any of the possible orientations. In oneembodiment, the reference material includes C₆₀.

Referring to block S3 of FIG. 2, EELS spectrum imaging is performed on apreselected area of the reference material and a preselected area of thecarbon overcoat to generate a reference EELS spectrum imaging datasetand a carbon overcoat EELS spectrum imaging dataset, respectively. Inmore detail, the EELS spectrum imaging includes performing a high energyresolution (e.g., about 1 eV) low-loss EELS spectrum imaging andcore-loss EELS spectrum imaging for both the area of the referencematerial and the area of the carbon overcoat.

In block S4 of FIG. 2, background subtraction is performed on thereference EELS dataset and the carbon overcoat EELS dataset to removethe background from the EELS datasets to extract signals for both thereference material and the carbon overcoat. The background of an EELSspectrum can originate from multiple inelastic electron scatterings andextension of previous absorption edges. In one embodiment, backgroundsubtraction is performed by fitting a power-law function to the observedbackground. However, the present invention is not limited thereto. Inseveral embodiments, other suitable background subtraction methods suchas a differentiation method can be used.

In block S5 of FIG. 2, Fourier-ratio deconvolution is performed toremove plural scattering, which can be generated by electrons thatundergo multiple inelastic scattering events primarily due to the largerphysical thickness than the mean free path of the inelastic scattering,for both the reference material and the carbon overcoat so that thereference EELS dataset and the carbon overcoat EELS dataset can befurther processed irrespective of the difference in thickness betweenthe reference material and the carbon overcoat. However, the presentinvention is not limited to using Fourier-ratio deconvolution. Inseveral embodiments, other suitable methods such as Fourier-logdeconvolution can be used to account for the thicknesses of thereference material and the carbon overcoat.

In block S6 of FIG. 2, the π* and σ* peak intensities (I_(π) and I_(σ))are determined or measured at the carbon K-edge for both the referencematerial and the carbon overcoat using, for example, a non-linear leastsquares (NLLS) fitting method according to an embodiment of the presentinvention. However, the present invention is not limited thereto. Inseveral embodiments, other suitable fitting methods can be used.Referring back to FIG. 1, the EELS spectra are fitted with Gaussiancurves by NLLS for determining the peak intensities (I_(π) and I_(σ))according to an embodiment of the present invention. The peakintensities (I_(π) and I_(σ)) can be determined according to Equations(3) and (4).I _(π)=(A)_(π)×(FWHM)_(π)  (3)I _(σ)=(A)_(σ)×(FWHM)_(σ)  (4)

In Equations (3) and (4), A is the amplitude of the correspondingGaussian curve, and FWHM is the full width at half maximum of theGaussian curve. The processes of block S6 are performed on both thecarbon overcoat EELS dataset and reference EELS dataset.

In block S7, the number fraction of sp³ bonded atoms and the numberfraction of sp² bonded atoms of the carbon overcoat can be determinedusing the above-described Equations (1) and (2) using the peakintensities (I_(π) and I_(σ)) determined in block S6. The processesdescribed in reference to blocks S2 through S7 provide a method forquantifying the sp³ carbon and sp² carbon content of the carbon overcoataccording to an embodiment of the present invention. Therefore, thesp³/sp² ratio of the carbon overcoat can be determined.

In block S8, the composition profile and two-dimensional map of thecarbon overcoat are extracted in the core-loss EELS spectrum from thecarbon overcoat EELS dataset. In one embodiment, the information can beextracted using suitable software. In one embodiment, software soldunder the trademark DigitalMicrograph®, which is made by Gatan, Inc. ofPleasanton in California, can be used to control the spectrometer, dataacquisition, and data processing.

The above described processes can be used to fully characterize a carbonovercoat having a thickness of 3 nm or less. In several embodiments, thedisclosed processes can be used to fully characterize a carbon overcoatof magnetic head or media. In other embodiments, the disclosed processescan be used to characterize a carbon overcoat at device level such aswafer, slider, and HGA. Therefore, the processes of the presentinvention can be used to evaluate and compare carbon overcoatsquantitatively in ultra thin carbon overcoat development andmanufacturing.

FIG. 3 is a flowchart illustrating a method for characterizing a carbonovercoat according to an embodiment of the present invention. In blockM1 of FIG. 3, EELS spectrum imaging is performed for an area of apreselected carbon-based material and an area of a carbon overcoat togenerate a reference EELS dataset and a carbon overcoat EELS dataset,respectively. In block M2 of FIG. 3, a carbon bonding content of thecarbon overcoat is determined using the reference EELS dataset and thecarbon overcoat EELS dataset. In several embodiments, some or all of theprocesses described in reference to FIG. 2 can be applied in block M2 todetermine the carbon bonding content of the carbon overcoat.

In the above described embodiments, the process or method can performthe sequence of actions in a different order. In another embodiment, theprocess or method can skip one or more of the actions. In otherembodiments, one or more of the actions are performed simultaneously orconcurrently. In some embodiments, additional actions can be performed.

While the present invention has been particularly shown and describedwith reference to embodiments, it will be understood by those ofordinary skill in the art that various changes in form and details maybe made therein without departing from the spirit and scope of thepresent invention as defined by the following claims and theirequivalents.

What is claimed is:
 1. A method for characterizing a carbon overcoatwith a thickness of three nanometers or less, comprising: performingelectron energy loss spectroscopy (EELS) spectrum imaging for an area ofa preselected carbon-based material and an area of a carbon overcoat ofa magnetic head or a magnetic data storage medium to generate areference EELS dataset and a carbon overcoat EELS dataset, respectively,wherein the carbon overcoat has a thickness of three nanometers or less,wherein the performing EELS spectrum imaging comprises performing highenergy resolution low-loss EELS spectrum imaging and core-loss EELSspectrum imaging for both the area of the preselected carbon-basedmaterial and the area of the carbon overcoat; removing plural scatteringin both the reference EELS dataset and the carbon overcoat EELS datasetto account for the difference in thickness between the preselectedcarbon-based material and the carbon overcoat; quantifying a ratio ofsp³ bonding to sp² bonding of the carbon overcoat based on the referenceEELS dataset and the carbon overcoat EELS dataset; and extracting acarbon overcoat element profile and a two-dimensional map of the carbonovercoat in the core-loss EELS spectrum from the carbon overcoat EELSdataset.
 2. The method of claim 1, further comprising measuring athickness of the carbon overcoat using a microscopy technique selectedfrom the group consisting of transmission electron microscopy (TEM) andscanning transmission electron microscopy (STEM).
 3. The method of claim2, further comprising measuring a thickness of a sub-layer of the carbonovercoat, the sub-layer comprising carbon and a seed material.
 4. Themethod of claim 3, wherein the thickness of the sub-layer of the carbonovercoat is measured using a microscopy technique selected from thegroup consisting of energy filtered transmission electron microscopy(EFTEM) and scanning transmission electron microscopy (STEM).
 5. Themethod of claim 1, wherein the preselected carbon-based materialcomprises a material selected from the group consisting ofmicrocrystalline graphite and C₆₀.
 6. The method of claim 1, wherein thepreselected carbon-based material comprises substantially 100 percentsp² carbon bonding.
 7. The method of claim 1, further comprisingperforming power-law background subtraction on the reference EELSdataset and the carbon overcoat EELS dataset to extract signals for boththe preselected carbon-based material and the carbon overcoat.
 8. Themethod of claim 1, wherein the quantifying the ratio of sp³ bonding tosp² bonding of the carbon overcoat comprises: determining a pi orbitalintensity (I_(π)) and a sigma orbital intensity (I_(σ)) at a carbonK-edge for both the carbon overcoat and the preselected carbon-basedmaterial using the EELS datasets; and calculating a number fraction ofsp³ bonding and a number fraction of sp² bonding of the carbon overcoatbased on the following equations:${{F\left( {sp}^{3} \right)} = \frac{1 - {3 \cdot \frac{N_{\pi}}{N_{\sigma}}}}{1 + \frac{N_{\pi}}{N_{\sigma}}}},{where}$${\frac{N_{\pi}}{N_{\sigma}} = \frac{\left\lbrack {I_{\pi}/I_{\sigma}} \right\rbrack_{coc}}{3 \cdot \left\lbrack {I_{\pi}/I_{\sigma}} \right\rbrack_{reference}}},{and}$F(sp²) = 1 − F(sp³), where F(sp³) is the number fraction of the sp³bonding, F(sp²) is the number fraction of the sp² bonding, N_(π) is anumber of carbon atoms having the sp² bonding, and N_(σ) is a number ofcarbon atoms having the sp³ bonding.
 9. The method of claim 8, whereinthe quantifying the ratio of sp³ bonding to sp² bonding of the carbonovercoat further comprises removing plural scattering by Fourier-Ratioin both the reference EELS dataset and the carbon overcoat EELS dataset.10. The method of claim 8; wherein the quantifying the ratio of sp³bonding to sp² bonding of the carbon overcoat further comprises applyinga non-linear least squares (NLLS) fitting on both the reference EELSdataset and the carbon overcoat dataset to determine the I_(π) andI_(σ).
 11. The method of claim 1, wherein the carbon overcoat comprisesa diamond-like carbon.
 12. A method for characterizing a carbon overcoatwith a thickness of three nanometers or less, comprising: measuring athickness of a carbon overcoat of a magnetic head or a magnetic datastorage medium using transmission electron microscopy (TEM), wherein thecarbon overcoat has a thickness of three nanometers or less; measuring athickness of a sub-layer of the carbon overcoat using energy filteredtransmission electron microscopy (EFTEM) or scanning transmissionelectron microscopy (STEM), the sub-layer comprising carbon and a seedmaterial; performing electron energy loss spectroscopy (EELS) spectrumimaging for an area of a preselected carbon-based material and an areaof the carbon overcoat to generate a reference EELS dataset and a carbonovercoat EELS dataset, wherein the performing EELS spectrum imagingcomprises performing high energy resolution low-loss EELS spectrumimaging and core-loss EELS spectrum imaging for both the area of thepreselected carbon-based material and the area of the carbon overcoat;removing plural scattering in both the reference EELS dataset and thecarbon overcoat EELS dataset to account for the difference in thicknessbetween the reference preselected carbon-based material and the carbonovercoat; quantifying a ratio of sp³ bonding to sp² bonding of thecarbon overcoat based on the reference EELS dataset and the carbonovercoat EELS dataset; and extracting a carbon overcoat element profileand a two-dimensional map of the carbon overcoat in the core-loss EELSspectrum from the carbon overcoat EELS dataset.
 13. The method of claim12, wherein the TEM is scanning transmission electron microscopy (STEM).14. The method of claim 12, wherein the quantifying a ratio of sp³bonding to sp² bonding of the carbon overcoat comprises: determining api orbital intensity (I_(π)) and a sigma orbital intensity (I_(σ)) at acarbon K-edge for both the carbon overcoat and the preselectedcarbon-based material using the EELS datasets; and calculating a numberfraction of the sp³ bonding and a number fraction of the sp² bonding ofthe carbon overcoat based on the following equations:${{F\left( {sp}^{3} \right)} = \frac{1 - {3 \cdot \frac{N_{\pi}}{N_{\sigma}}}}{1 + \frac{N_{\pi}}{N_{\sigma}}}},{where}$${\frac{N_{\pi}}{N_{\sigma}} = \frac{\left\lbrack {I_{\pi}/I_{\sigma}} \right\rbrack_{coc}}{3 \cdot \left\lbrack {I_{\pi}/I_{\sigma}} \right\rbrack_{reference}}},{and}$F(sp²) = 1 − F(sp³), where F(sp³) is the number fraction of the sp³bonding, F(sp²) is the number fraction of the sp² bonding N_(π) is anumber of carbon atoms having the sp² bonding, and N_(σ) is a number ofcarbon atoms having the sp³ bonding.